1. Field of the Invention
The present invention generally relates to a mobile communication system, and more particularly, to a mobile communication system that is capable of switching the duplex modes of an uplink or downlink depending on the transmission volume communicated between a base station and a mobile station.
2. Description of the Related Art
Duplexing carried out in the current mobile communication system is generally classified into frequency division duplex (FDD) and time division duplex (TDD).
FIG. 1 illustrates a conventional cell structure in FDD communication, and FIG. 2 illustrates the basic concept of conventional FDD communication. As shown in FIG. 1, base stations 111, 112, and 113 are located in the respective cells 400, 500, and 600, and connected to mobile stations 211, 212, and 213, respectively, via radio channels. In FDD, different frequencies f1 and f2 are allocated to the uplink, which is directed from the mobile station to the base station, and the downlink, which is directed from the base station to the mobile station. The radio frequencies f1 and f2 occupy the associated links all the time, as shown in FIG. 2. In addition, the radio frequencies f1 and f2 are used equally in all the cells 400, 500, and 600.
FIG. 3 illustrates a conventional FDD mobile communication system. This FDD system includes a base station 111 and a mobile station 211. The base station 111 has a signal processing unit 80 connected to the backbone network (e.g., mobile communication network), a transmitter 81, a receiver 82, a band pass filter (BPF) 83 with a center frequency of f1, a band pass filter (BPF) 84 with a center frequency of f2, and antennas 85, 86 connected to the receiver 82 and the transmitter 81, respectively.
The mobile station 211 has an input unit 70 (e.g., microphone 70a or keyboard 70b), an output unit 71 (e.g., display 71a or speaker 71b), a signal processing unit 72, a transmitter 73, a receiver 74, a band pass filter (BPF) 75 with a center frequency of f1, a band pass filter (BPF) 76 with a center frequency of f2, and antennas 77, 78 connected to the transmitter 73 and the receiver 74, respectively.
In operation of the mobile station 211, digital signals supplied from the input unit 70 are converted to baseband signals by the signal processing unit 72. The baseband signals are converted to radio signals by the transmitter 73, which then pass through the band pass filter 75. Radio signal components of center frequency f1 are extracted by the band pass filter 75, and the signals are transmitted from the antenna 77 to the base station 1 at a frequency f1.
The radio signals transmitted from the mobile station 211 are received by the antenna 85 of the base station 111, and supplied to the receiver 82 via the band pass filter 83. The received radio signals are converted to baseband signals, and the baseband signals are then converted to digital signals by the signal processing unit 80. The digital signals are transmitted to the backbone network, such as a mobile communication network.
On the other hand, in the downlink, digital signals received by the base station 111 from the backbone network are converted to baseband signals by the signal processing unit 80, and then converted to radio signals by the transmitter 81. The band pass filter 84 extracts only those signal components with a center frequency of f2, which are then transmitted to the mobile station 211.
The radio signals transmitted from the base station 111 are received by the antenna 78 of the mobile station 211, and supplied to the receiver 74 via the band pass filter 76. The radio signals are converted into digital signals through the receiver 74 and the signal processing unit 72, and supplied to the output unit 71.
In FDD, radio communication is carried out by allocating different frequencies f1 and f2 to the uplink (from the mobile station 211 to the base station 111) and the downlink (from the base station 111 to the mobile station 211.
FIGS. 4-7 illustrate the concept of conventional TDD communication. In conventional TDD, the same radio frequency is used on the uplink and the downlink, unlike FDD. As illustrated in FIG. 4, base stations 121-1214 are located in the respective cells 700-713. In this cell structure, a mobile station 221, which is under management of the base station 121, is connected to a mobile station 222, which is under management of the base station 122 by means of TDD communication. In TDD, three or more frequencies (f1-f3) are allocated alternately and repeatedly to the cells (#1-#3 ). This arrangement is required to guarantee a sufficient spatial distance between those cells that use the same radio frequency in order to preclude interference from the neighborhood cells. Each frequency allocated to one of the cells #1-#3 is used for the uplink and the downlink alternately each at an equal time interval. The time period allocated to the alternate links is called a time slot.
FIG. 6 illustrates a conventional TDD mobile communication system. This TDD system includes a base station 121 and a mobile station 221. The base station 121 has a signal processing unit 120 connected to the backbone network (e.g., mobile communication network), a transmitter 122, a receiver 123, a switch (SW) 124, a band pass filter (BPF) 125 with a center frequency of f1, and an antenna 126 for transmitting and receiving radio signals.
The mobile station 221 has an input unit 110, such as a microphone 110a or a keyboard 110b, an output unit 111, such as a display 111a or a speaker 111b, a signal processing unit 112, a transmitter 114, a receiver 115, a switch (SW) 116, a band pass filter (BPF) 117 with a center frequency of f1, and an antenna 118 for transmitting and receiving radio signals.
In operation of the mobile station 221 in the TDD system, digital signals supplied from the input unit 110 are converted to baseband signals by the signal processing unit 112. The baseband signals are converted to radio signals by the transmitter 114, which then pass through the switch (SW) 116 and the band pass filter (BPF) 117. Radio signal components of center frequency of flare extracted, and are transmitted from the antenna 118 to the base station 121 at a frequency f1.
The radio signals transmitted from the mobile station 221 are received by the antenna 126 of the base station 121, and supplied to the receiver 123 via the band pass filter (BPF) 125 and the switch (SW) 124. The received radio signals are converted to digital signals by the receiver 123 and the signal processing unit 120. The digital signals are then transmitted to the backbone network.
On the other hand, in the downlink, digital signals received at the base station 121 from the backbone network are converted to radio signals by the signal processing unit 120 and the transmitter 122. The radio signals pass through the switch (SW) 124 and the band pass filter (BPF) 125, in which only those signal components with a center frequency of f1 are extracted. The radio signals of frequency f1 are transmitted from the antenna 126 to the mobile station 221.
The radio signals transmitted from the base station 121 are received by the antenna 118 of the mobile station 221, supplied to the receiver 115 via the band pass filter (BPF) 117 and the switch (SW) 116. The radio signals are converted into digital signals through the receiver 115 and the signal processing unit 112, and supplied to the output unit 111.
The switching controllers 121 and 113 of the base station 121 and the mobile station 221 respectively, superpose control signals on the downlink communication signals so that the base station 121 and the mobile station 221 synchronize with each other. The switching controllers 121 and 113 switch the switches (SWs) 124 and 116, respectively, based on the control signal supplied from the signal processing unit 120 of the base station.
With this arrangement, during uplink communication, the signal processing unit 120 of the base station 121 stops supplying signals to the transmitter 122. Accordingly, the transmitter 122 of the base station 121 and the receiver 115 of the mobile station 221 suspend operations. On the other hand, during the downlink communication, the signal processing unit 112 of the mobile station 221 stops supplying signals to the transmitter 114. Accordingly, the transmitter 114 of the mobile station 221 and the receiver 123 of the mobile station 121 suspend operations.
In recent years, the volume of data transmitted in mobile radio communication has increased greatly. Unlike voice communication on telephone lines, data communication is likely to produce asymmetric situations in the volume of data between the uplink and the downlink. In particular, as browser phones, which are cellular phones allowing the users to access homepages uploaded in servers on the Internet or use electronic mail, have rapidly spread, digital contents is frequently downloaded from the servers set by providers. As a result, the data amount transmitted on the downlink is likely to be larger than that on the uplink.
In the conventional duplex method (either FDD or TDD), even if the data amount on the downlink is much larger than that on the uplink because of asymmetric transmission volume, an equal radio frequency or a time slot has to be allocated to the uplink and the downlink. For this reason, the radio resources, such as a time slot or a frequency band, are wasted. Besides, since the radio frequency band allocated to mobile radio communication is limited, the transmission capacity of the system is also restricted.
Accordingly, it is an important requirement to eliminate waste of radio resources so as to maximize deterioration of the transmission capacity of the system in the limited radio frequency band. From this viewpoint, the 3rd Generation Partnership Project (3GPP), which is an organization that investigates technological standards of IMT-2000, has proposed asymmetric duplex as illustrated in FIG. 7.
In this method, a series of frames (F#1, F#2, . . . F#n), each consisting of a plurality of time slots (for example, four in the example of FIG. 7), are provided. The numbers of time slots allocated to the uplink and the downlink are set asymmetric in order to cope with asymmetric data volume on the uplink and the downlink.
However, this asymmetric duplexing is unsuitable to a large-capacity mobile communication system because data are transmitted in the same radio frequency band in both the uplink and the downlink, and therefore, a continuing wide range of frequency bands must be guaranteed. Unfortunately, the allocated radio frequency bands are overcrowded, and it is difficult to obtain a continuing wide range of frequency bands.
Another problem arises if TDD is applied to WCDMA systems, and if the data volumes in the uplink and the downlink become asymmetric. In asymmetric TDD-type ACDMA, as the volume of the communication data increases, high-speed processing is required in generating clock frequencies for the baseband signal and modulating the baseband signal to a radio signal. This causes the structures of both the mobile terminal and the base station to be complicated.